Electromagnet



April 24, 1951 PETERS 2,550,123

ELECTROMAGNET Filed Oct. 1, 1949 l I l WITNESSES: INVENTOR 2 0 John F.Peters.

27 BY MMW W; W

ATTORNEY invention.

Patented Apr. 24, 1951 ELECTROMAGNET John F. Peters, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 1, 1949, Serial No. 119,078

4 Claims.

This invention relates generally to electromagnetic devices and more in particular to an electromagnetic device which may be employed as an actuator for a control means.

Electromagnetic devices are frequently employed as actuating devices in various types of control systems. When so employed, these electromagnetic devices are energized by electrical quantities representative of an error in a quantity which is to be controlled and the output motion of the armature member of the electro magnetic devices are utilized through suitable instrumentalities to control the quantity which is to be controlled.

In such applications it is frequently desirable that the output force of the armature member of the electromagnetic device shall bear a linear relationship with the quantity that is to be controlled. A conventional electromagnetic device is not suitable in effecting such a control inasmuch as the biasing force of the armature member thereof varies with respect to its displacement with respect to the core. This variation is an inverse function of the displacement, that is, the smaller the gap dimension between the armature and the core becomes, the larger the magnetic pull on the armature becomes. This results in a condition which is frequently referred to as negative stiffness and various means have been employed in the past to overcome this condition.

One object of this invention is to provide an electromagnetic device which is simple in its elements and positive in operation.

Another object of this invention is to provide an electromagnetic device having a core and a movable armature in which the magnetic force acting on the armature is independent of the armature displacement with respect to the core.

Yet another object of this invention is to provide an electromagnetic device of the character referred to in which the armature force may be made a linear function of the coil current.

The foregoing statements are merely illustrative of the various aims and objects of this Other objects and advantages will become apparent upon a study of the following disclosure when considered in conjunction with the accompanying drawing in which:

Figure l is an elevational view of an electromagnetic device embodying the principles of this invention, and

Figure 2 is a sectional view taken on the line II-II of Figure 1.

The electromagnetic device illustrated in the two figures of the drawing includes a pair of opposed electromagnetic members generally designated I and 2, respectively. The complete assembly includes a common armature assembly 3 which cooperates with both of the electromagnetic members I and 2.

More in particular the electromagnetic members and 2 include, respectively, rectangular core structures 4 and 5 which are formed of suitable laminations of magnetic material. The laminated structure of the core 5 appears in Figure 2. Coils 6 and 1 are disposed about cor responding legs 8 and 9 of the respectiveelectromagnetic core members 4 and 5. The positioning of these coils is such that upon excitation thereof the magnetic field produced by the coils links the respective cores. Each of the cores 4 and 5 is provided with an arcuate air gap. These are respectively designated in and H. The air gaps are defined by arcs having equal radii and the core members are positioned in end to end relation and supported in this position by means of plates !2 of non-magnetic material which are disposed on opposite sides of the core structure. These plates I2 space the corresponding legs I3 and M respectively of the cores 4 and 5 a predetermined amount to minimize stray flux linkage between the core members and in this position the air gaps Iii and H define a single arc. The center of curvature of this are is coincident with the axis of a shaft [5 between the core assemblies which shaft functions as the pivot for the armature assembly 3 (see Fig. 2).

Referring particularly to Figure 2, the shaft I5 is disposed within a spacing sleeve I6 which in turn is journalled in bearings I! carried in the oppositely disposed plates l2. The sleeve is is sufficiently longer than the dimension across the outer faces of the bearings I! that the washers I8 which slide over each end of the shaft IS in abutting relation with the ends of the spacing sleeve IE will not bind upon the adjacent faces of the bearings IT.

The armature assembly is of U-shaped configuration and is comprised of a U-shaped bracket 59, which is formed of non-magnetic material of channel shaped cross section. The sides of the channel are removed across the bight of the U- shaped bracket and, as will be seen in Figure 2, the legs 2E1 of the U-shaped bracket are preferably of channel shape cross section to stiffen the assembly. The armature member 2| is of magnetic material and is of a suitable length to pass from one side of the core section to the other. The width of the armature member 2| is sufiicient when the armature assembly is centered as shown in Figure 1 that the transverse edges of the armature project into the respective arcuate air gaps l and H in spaced relation with the sides thereof, as will be observed in Figure 1. The armature member is of arcuate shape to conform to the arc of the air gaps l0 and H. Thus, as the armature member is moved clockwise or counterclockwise, as viewed in Figure 1, one side of the armature will project further into the corresponding air gap while the other side of the armature is removed from its associated air gap in a corresponding amount. As a consequence, the effective area, of the air gap on one side of the armature is increased while the efiective area of the air gap on the other side is decreased. It will also be observed in this movement that the radial dimensions of the air gaps remain unchanged. As a consequence there is no change in reluctance across the air gaps due to a change in gap dimension, the only change in reluctance being due to the change in the area of the gap. The armature member is laminated between the bight of the U-shaped bracket 29 and the bight of a U-shaped bracket 24, the lamination being secured by rivets 22 and a centrally disposed pin 23. The legs of bracket 24 are riveted to the legs of the U-shaped bracket 28 to complete the assembly. Extensions of the legs of the U-shaped bracket 2!] extend below the shaft H) as viewed in both Figures 1 and 2 and adjacent the extremities thereof are fitted with bearings 25 and 26 which journal a shaft 21 supporting a push-pull rod 28 centrally thereof. Rod 28 is employed to actuate a suitable control means, for example, the operating member of a hydraulic valve.

In practice the armature assembly need not move through an appreciable dimension. In the suggested valve application the angular displacement of the armature assembly, for example, need be no more than one or two degrees. In any event the motion of the armature is limited in any convenient manner so that the armature is never moved completely from one of the air gaps.

With the construction illustrated it will be appreciated that the magnetic force where L is inductance expressible in terms of flux lines per ampere, X is equal to armature travel, and I is equal to the current which is utilized to excite the coils. The flux density is proportional to X. The differential of L with respect to X is, therefore, constant and the force on the armature is proportional to I only. This analysis views the biasing effects of the two electromagnets on the armature separately.

In the physical embodiment illustrated the magnetic force of the electromagnet l upon the armature opposes the magnetic force of electromagnet 2 upon the same armature. In such an arrangement the force may be expressed as a function of the difference of the squares of coil current, that is,

(Li in which the current IA represents the current of coil 1 and the current In represents the current of coil 6. When the coils are fed from a suitable supply means of electrical energy so that IA+Is:2Io, a constant, then IA=Io+i and 13:10-11, wherein i represents the control current which is applied to both of the coils. In one case the control current i is additive and in the other case subtractive. The force F is expressed as: F=K[(Io+i) (I0-i) l=4KIoi. Thus, by completing the squares it will be appreciated that a quantity 4KI02' is obtainable which is equal to the armature force. Since 4KI0 is a constant, the armature force is, therefore, proportional to the first power of the control current and is also independent of the position of the armature with respect to the core.

Occasionally, a control arrangement may require a means for mechanically centering the armature assembly 3. This may be accomplished by means of springs 29 and 30 which are connected, respectively, between the pin 23 connected to the armature and the pins 3| and 32, respectively, secured to cores I and 2. These springs may also serve an additional purpose. Assume, for example, that the push-pull control rod 28 is connected to an operating member of a valve in which a force acting on the operating member when displaced tends to bias the armature memher in the direction of the displacement. In such an instance the springs 29 and 3E! produce a restoring force tending to compensate the force applied to the armature through the rod 28. The magnet response and its position is thus more accurately indicative of the control current supplied thereto.

In view of the foregoing considerations it will be appreciated that this invention provides a simple and effective electromagnetic device in which the armature force may be readily made a linear function of the coil current.

Although but one embodiment of this invention has been illustrated it will be appreciated that numerous variations in the details of the structure may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the foregoing disclosure and the illustrations in the drawing are to be considered only as illustrative of the principles of this invention and are not to be interpreted in a limiting sense.

I claim as my invention:

1. In an electromagnetic device, the combination of, a pair of core members each having four legs and each having an arcuate air gap therein in corresponding legs, non-magnetic supporting means positioning said core members in spaced side by side relation, said arcuate air gaps defining an are having its center point substantially on the center line between said core members, an armature assembly pivotally mounted on said supporting means at said center point and having an arcuate armature member of magnetic material, said armature member being supported between said core members with its opposite sides projecting into said arcuate air gaps in spaced relation with the sides thereof, and means for exciting said core members.

2. In an electromagnetic device, the combination of, a pair of core members each having four legs and each having an arcuate air gap therein in corresponding legs, non-magnetic supporting means positioning said core members in spaced side by side relation, said arcuate air gaps defining an are having its center point substantially on the center line between said core members, an armature assembly pivotally mounted on said supporting means at center point and having an arcuate armature member of magnetic material, said armature member being supported between said core members with its opposite sides projecting into said arcuate air gaps in spaced relation with the sides thereof,

centering springs secured between said armature assembly and said core members for substantially centering said armature assembly between said core members, and means for exciting said core members.

3. In an electromagnetic device comprising, in combination, a pair of core members each having four legs and each having arcuate air gaps of equal radii in corresponding legs, non-magnetic supporting means positioning said core members in spaced side by side relation with said arcuate air gaps defining a single arc, the spacing of said core members being suflicient to minimize the linkage of stray flux of one core member with the other core member, an arcuate armature member of magnetic material straddling the distance between said core members and being of a length suflicient to project into both of said arcuate air gaps, means supporting said armature member for movement along the arc defined by said arcuate air gaps in spaced relation with the sides of each thereof, and coil means for exciting said core members.

4. In an electromagnetic device comprising, in combination, a pair of core members each having four legs and each having arcuate air gaps of equal radii in corresponding legs, non-magnetic supporting means positioning said core members in side by side relation with said arcuate air gaps defining a single arc, the spacing of said core member being suflicient to minimize the linkage of stray flux of one core member with the other core member, an arcuate armature member of magnetic material straddling the distance between said core members and being of a length sufilcient to project into both of said arcuate air gaps, means supporting said armature member for movement along the arc defined by said arcuate air gaps in spaced relation with the sides of each thereof, supporting means secured between said armature member and said core members for substantially centering said armature member between said core members so that the-portions of said armature member projecting into both of said air gaps are equal, and coil means for exciting said core members.

JOHN F. PETERS.

REFERENCES CITED The following referencesare of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 178,580 Allan et a1 June 13, 1876 479,828 Mumford Aug. 2, 1892 719,325 Gray Jan. 27, 1903 800,654 Kitsee Oct. 3, 1905 811,539 Bates Feb. 6, 1906 928,582 Burlingame July 20, 1909 FOREIGN PATENTS Number Country Date 571,723 Germany Nov. 10, 1931 64,298 Denmark Feb. 18, 1946 

